1. Field of the Invention
The present invention relates to the coating of high temperature superalloys, such as high nickel- and high cobalt-superalloys to provide them with an improved protective outer layer which has increased and stabilized resistance to oxidation and corrosion when subjected to such atmospheres at high temperatures by virtue of increased ductility and stabilization of the Beta NiAl and Beta CoAl phases. An important use of such superalloys is for turbine blades in jet aircraft or power generation engines which perform at high temperatures and in corrosive and oxidizing atmospheres.
2. Discussion of the Prior Art
It is known to form protective coatings on the surface of metal superalloy components, such as turbine blades, using metals to form layers which are more resistant to corrosion and/or oxidation at high temperatures than is the base superalloy.
According to one such procedure, disclosed in U.S. Pat. No. 3,677,789 by Bungardt et al., the base superalloy is first coated with a thin layer of noble metal, such as platinum, and is then subjected to a high temperature aluminum diffusion treatment for several hours to form a protective layer diffused into and integrated with the base superalloy. The formed protective surface layer comprises platinum aluminide which has the disadvantages of being brittle, subject to craze cracking and has low impact strength.
Diffusion coating compositions and procedures for diffusing both aluminum and chromium into superalloy base metal components in a single step are known from U.S. Pat. No. 4,293,338 by Rose et al. Thus, the prepared superalloy base component is packed into a conventional diffusion-coating container together with a powdered cementation pack coating composition containing intermetallic CO.sub.2 Al.sub.9 powder and chromium metal powder, heated to about 1925.degree.-1975.degree. F. for about three-four hours in a hydrogen gas atmosphere, removed and post-treated in a hydrogen, inert gas or vacuum atmosphere for about two hours at about 1950.degree. F.-1975.degree.. A codeposited diffusion layer of aluminum and chromium is thereby provided at the superalloy metal surface, but in the absence of any platinum group metal.
According to another known procedure, disclosed in U.S. Pat. No. 4,526,814 by Shankar et al., protective diffusion layers of a platinum group metal, chromium and aluminum are formed at the surface of superalloy base components in a multi-step process in which the superalloy base component is first coated with the platinum group metal, post-platinized at about 1900.degree. F. for three hours to diffuse the platinum metal into the superalloy, then high temperature-chromized at 1950.degree. F. for eight hours to form a diffusion layer of the platinum group metal and chromium into the superalloy. The platinized metal is then high temperature-aluminized at about 1400.degree. F. for five hours, to form a diffusion layer of the platinum group metal, chromium and aluminum into the superalloy base surface. Next a post-coating diffusion treatment is done at 1975.degree. F. for 2 to 4 hours. Such procedure is tedious and expensive because of the several steps including the post-platinizing heating step and the pre-aluminizing heating step. Also, in cases where low amounts of chromium are diffused into the protective layer, the layer is limited in effectiveness of protection to high temperature oxidation and high temperature hot corrosion application (2000.degree. F.-1700.degree. F.).
Previously, increased ductility in a platinum improved aluminide was achieved by beta chromium ductilization of a platinum group metal coated surface. This is known as RB582, and employs the methodology of chromium-aluminum deposition known as RB505 (U.S. Pat. No. 4,293,338), and is disclosed in commonly owned U.S. patent application Ser. No. 875,375 filed Apr. 29, 1992, now abandoned but refiled as Ser. No. 212,274, filed Mar. 14, 1994, pending. The RB582 process is an improved and simplified process compared to that taught by Shanker et al. in U.S. Pat. No. 4,526,814.
It is also known to codeposit small amounts of silicon, with aluminum and chromium, in order to regulate the amount of aluminum which deposits and thereby increase the amount of chromium which can deposit, thereby improving the stability of the deposit against penetration into the substrate. Reference is made to U.S. Pat. No. 4,055,706 (Galmiche et al) and U.S. Pat. No. 4,528,215 (Boldi et al).